Create DES key from 56 bit binary string

java binary jca
AnkurVj picture AnkurVj · Feb 13, 2011 · Viewed 9.3k times · Source

I have a 56 bit binary string that i want to use as the secret key for DES encryption.

I found the following code at the JCA docs website 

byte[] desKeyData = { (byte)0x01, (byte)0x02, (byte)0x03, 
(byte)0x04, (byte)0x05, (byte)0x06, (byte)0x07, (byte)0x08 };
DESKeySpec desKeySpec = new DESKeySpec(desKeyData);
SecretKeyFactory keyFactory = SecretKeyFactory.getInstance("DES");
SecretKey secretKey = keyFactory.generateSecret(desKeySpec);

However this uses 8 bytes for the key (instead of 7). It is not clear if the desKeyData[0] corresponds to the least significant byte or the most significant one. Also, is it possible to use the 56 bit string directly to generate the byte array that can be used for this purpose ?

Answer

axtavt picture axtavt · Feb 13, 2011

From Wikipedia:

The key ostensibly consists of 64 bits; however, only 56 of these are actually used by the algorithm. Eight bits are used solely for checking parity, and are thereafter discarded. Hence the effective key length is 56 bits, and it is never quoted as such. Every 8th bit of the selected key is discarded, i.e. positions 8, 16, 24, 32, 40, 48, 56, 64 are removed from the 64 bit key leaving behind only the 56 bit key.

So, the least significant bits (i.e. 0th bits) are not used for key construction, they can be used for checking parity by DESKeySpec.isParityAdjusted().

EDIT: Simple test showing that the least significant bits are ignored:

SecretKeyFactory sf = SecretKeyFactory.getInstance("DES");
byte[] in = "test".getBytes("UTF-8");

Cipher c1 = Cipher.getInstance("DES");
c1.init(Cipher.ENCRYPT_MODE, sf.generateSecret(new DESKeySpec(
   new byte[] {0x10,0x20,0x30,0x40,0x50,0x60,0x70,(byte) 0x80})));
byte[] r1 = c1.doFinal(in);

Cipher c2 = Cipher.getInstance("DES");
c2.init(Cipher.ENCRYPT_MODE, sf.generateSecret(new DESKeySpec(
    new byte[] {0x11,0x21,0x31,0x41,0x51,0x61,0x71,(byte) 0x81})));
byte[] r2 = c2.doFinal(in);

assertArrayEquals(r1, r2);

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